Electromagnetic well logging is an important technique used in the oil and gas industry. Logging tools to measure the resistivity or conductivity of subterranean earth formations adjacent a borehole have long been standard in the search for and recovery of hydrocarbons. Generally, a transmitter transmits an electromagnetic signal that passes through the formation around the borehole. These signals then induce a signal in one or more receivers located on the tool. The properties of the signals received after passing through the formation, such as amplitude and/or phase, allow well engineers to make resistivity or other measurements of the formation characteristics. The measurements and formation properties are then recorded as a function of the tool's depth or position in the borehole.
One important electromagnetic measurement is the determination of the dielectric constant of a downhole formation. A particularly suitable tool for this task is the High Frequency Dielectric Tool (“HFDT”) available from Halliburton Corp. The HFDT may be used to measure the apparent dielectric constant and resistivity of a downhole formation and obtain formation properties, such as the water-filled formation porosity. The collection of information relating to conditions downhole, referred to as “logging,” can be performed by several methods including wireline logging, logging while drilling (“LWD”), drillpipe conveyed logging, and coil tubing conveyed logging.
In wireline logging, a probe or “sonde” is lowered into the borehole after some or all of the well has been drilled. The sonde is suspended at the end of a cable or “wireline” that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In accordance with existing logging techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.
In LWD, the drilling assembly includes sensing instruments that measure various parameters in the formation as is it being drilled. LWD techniques provide more contemporaneous formation measurements, but the drilling operations create a more difficult operating environment.
In drillpipe or coil tubing conveyed logging, sensing instruments are mounted on a tubing string, which moves the instrument package through an existing borehole. The tubing string enables logging of horizontal well bores without requiring the sensing instruments to tolerate the hostile drilling environment. Typically, the measurement data is stored in internal memory and recovered along with the instrument package.
Dielectric tools determine the dielectric constant and conductivity of downhole formations from the real and imaginary parts of the complex propagation constant of electromagnetic waves traveling through the formations. By measuring the phase difference and amplitude ratio between two points in the formation, the tool determines the formation resistivity and dielectric constant.
To transmit and receive the electromagnetic signals necessary for taking measurements, downhole logging tools use antennas. These antennas may be coils or they may be cavity antennas. Existing tool designs have undesirable limitations regarding these antennas, however. For example, the geometry of cavity antennas may be limited by other constraints involved in the design of the tool. Coil antennas are subject to distortion of the electromagnetic signal caused by the steel making up the tool backbone.
Accordingly, what is needed are tools and methods to overcome these shortcomings.